Mars - located in Gemini and Cancer during May.  Mars will be 50 degrees up in the WSW by sunset at mid-month.  The "red" planet will fade from magnitude 1.2 to 1.5 and have an apparent diameter that decreases from 6 to 5 arc-seconds during the month.  Mars is still large enough to see the major surface features in an 8" telescope.  The Martian atmosphere is now clear of dust, allowing good views of the surface.  Besides the usual dark surface markings, the North Polar Cap should be visible.  Look for Mars in the northern edge of M44 (The Beehive Cluster) on the evenings of the 22nd and 23rd.  

Recent images of Mars can be seen here:

http://elvis.rowan.edu/marswatch/images.php

http://www.damianpeach.com/mars07.htm

http://astro.christone.net/mars/index.html

The Phoenix Lander will arrive at Mars on May 25, landing near the north polar region.  For the latest news and images, see:

http://phoenix.lpl.arizona.edu/

Saturn - located in Leo.  The ringed planet is currently well placed for observing for the first half of the night.  Look for it near 1st magnitude Regulus during May.  Saturn is just past the meridian by sunset at mid-month, when it will then be 60 degrees above the horizon.  Saturn will be magnitude 0.6 during May with an apparent diameter of 18 arc-seconds.  The rings will be 39 arc-seconds across and tilted 10 degrees to our line-of-sight.  By late December, the ring tilt will be only 1 degree.  At least 6 of Saturn's many moons are bright enough to be seen with an 8" scope.  

Jupiter - located in Sagittarius.  Jupiter rises around 1:00am EDT at mid-month and will be 27 degrees up in the south an hour before sunrise.  The "king of the planets" will be magnitude -2.5 with an apparent diameter that increases from 41 to 45 arc-seconds.  Jupiter's Great Red Spot, a large anti-cyclone, can be seen centered on the planet's disc at the following times (EDT): 

05/03/2008, 03:40 am; 05/05/2008, 05:18 am; 05/10/2008, 04:26 am; 05/12/2008, 06:04 am; 05/15/2008, 03:33 am; 05/17/2008, 05:11 am; 05/22/2008, 04:19 am; 05/24/2008, 05:57 am; 05/27/2008, 03:26 am; 05/29/2008, 05:04 am

Additional times for viewing the Great Red Spot can be found here: 

http://skytonight.com/observing/objects/planets/3304091.html

A guide for making visual observations of Jupiter can be found here: 

http://chagrinvalleyastronomy.org/Observing Jupiter.ppt

Mercury received its first visit from a spacecraft in 33 years on January 14th.  For the latest images and other details, see:

http://messenger.jhuapl.edu/